Quartz formation processes in the Icelandic crust were assessed using coupled δ18O and δ30Si systematics of silica deposits formed over a wide temperature range (<150 to >550 °C). Magmatic quartz reveals δ18O (-5.6 to +6.6 ‰) and δ30Si (-0.4 ± 0.2 ‰) values representative of mantle- and crustally-derived melts in Iceland. Hydrothermal quartz and silica polymorphs display a larger range of δ18O (-9.3 to +30.1 ‰) and δ30Si (-4.6 to +0.7 ‰) values. Isotope modelling reveals that such large variations are consistent with variable water sources and equilibrium isotope fractionation between fluids and quartz associated with secondary processes occurring in the crust, including fluid-rock interaction, boiling and cooling. In context of published δ18O and δ30Si data on hydrothermal silica deposits, we demonstrate that large ranges in δ30Si values coupled to insignificant δ18O variations may result from silica precipitation in a hydrothermal fluid conduit associated with near-surface cooling. While equilibrium isotope fractionation between fluids and quartz seems to prevail at high temperatures, kinetic fractionation likely influences isotope systematics at low temperatures.

Figure 3 The conceptual chemical and isotope model involves (a) fluid-rock interaction with various source fluids and (b) boiling followed by cooling and mineral formation; (c) isotope systematics in hydrothermal high-temperature quartz largely match the predicted reaction path for progressive fluid-rock interaction (shaded areas) and boiling (arrows). δ30Si decreases with increasing rock-fluid ratio (ξ) whereas δ18O values in quartz both depend on the source water and rock-fluid ratio; (d) most of the isotope variations in hydrothermal low-temperature quartz and silica polymorphs can be explained by boiling and cooling of a hydrothermal fluid (arrows). Silica deposits with low δ30Si and constant δ18O values may result from open system boiling of a high-temperature fluid followed by cooling and quartz or opal formation. BAS = basalt, mw = meteoric water, sw = seawater. Analytical uncertainties of δ30Si and δ18O are listed in Tables S-7 and S-8.

). Under magmatic conditions, quartz may crystallise at ~700 °C and numerous metamorphic reactions involve the consumption and production of quartz. Because of its common association with hydrothermal and ore deposits, the origin of quartz and its paragenesis continues to be a subject of considerable discussion.

Over the past decades, oxygen isotopes have been applied as a tracer for the source(s) of crustal fluids (e.g., Bowman et al., 1994

). However, Si isotopes have only gained interest recently due to improved analytical techniques. Silicon isotopes of quartz and other silica polymorphs range from -5 to +5 ‰ and have been used for reconstruction of past geological environments (Fig. 1). δ30Si systematics in Precambrian chert deposits have been used to constrain environmental conditions during their formation (e.g.,Robert and Chaussidon, 2006

). Such data are critical to constrain the processes (e.g., fluid-rock interaction) that lead to the formation of secondary quartz in the Earth’s crust, and can contribute to better understand the origin and formation conditions of ancient silica deposits.

Figure 1 Compilation of δ30Si values for different rock types and fluids. Data taken from Douthitt (1982)

), the Icelandic crust represents a continental-type crust with characteristics typical of the oceanic crust. Iceland’s crustal lithology is dominated by basalts with silicic volcanics and volcaniclastic sediments also being common (Sæmundsson, 1979

), fluid sources, chemical reactions and associated isotope fractionations were quantified for various processes occurring in the curst, including fluid-rock interaction, fluid phase separation (boiling) and cooling. Implications of this dataset and the isotope modelling were further extended to explain the origin of ancient hydrothermal silica deposits.

Oxygen isotopes in magmatic quartz display δ18O values of -5.6 to +6.6 ‰, while silicon isotopes show a very limited range of δ30Si values of -0.7 to -0.2 ‰ (Fig. 2). In contrast, both δ18O (-9.3 to +30.1 ‰) and δ30Si (-4.6 to +0.5 ‰) of hydrothermal quartz display a much greater range. Trace element concentrations of quartz agree with existing quartz data from other magmatic and hydrothermal settings (Götze, 2009

; Table S-2). Al and Ti in magmatic quartz range from 57 to 77 ppm and 56 to 140 ppm, respectively, while Al and Ti in hydrothermal quartz and silica polymorphs span a wide range (Al = 28-2140 ppm; Ti = 20-80 ppm). Additionally, trace element concentrations in hydrothermal quartz do not show any distinct correlation with δ18O and δ30Si that would allow us to further distinguish different formation conditions (Fig. S-3).

; this study). Silica and oxygen are both reactive elements and their isotope ratios in minerals and fluids may change significantly upon crustal processes and associated chemical reactions. The observed wide range in isotopic values of hydrothermal silica deposits highlights the need for quantification of such processes and their effects on the isotope systematics. Processes of importance in hydrothermal systems include fluid-rock interaction, phase separation (boiling) and temperature changes (cooling). Such processes lead to changes in the relative abundance of elements of various sources, e.g., the water to rock ratio, changes in aqueous species and gas concentrations and quantity of secondary minerals formed, which may all influence isotopic characteristics of fluids and minerals (Stefánsson et al., 2017

). However, as a result of extensive fluid-rock interaction involving a low-18O meteoric water component, strong δ18O-depletions from such a mantle source are commonly observed in Icelandic rocks (Muehlenbachs et al., 1974

). As a result, the δ30Si values of magmatic quartz (this study) likely represent magmatic values. This implies that the measured δ30Si values have not been affected by interaction of the xenoliths with meteoric water and/or other secondary processes.

The large range in δ18O and δ30Si values of hydrothermal quartz and silica polymorphs (Fig. 2) are considered to reflect the source(s) of the elements and isotope fractionations associated with hydrothermal processes and reactions. These processes and their effects on the δ18O and δ30Si values in quartz can be quantified using isotope modelling approaches (Fig. 3a,b; detailed description in SI).

, Fig. 3c,d). Upon progressive fluid-rock interaction, the silica is dissolved from the rock by the hydrothermal fluid and precipitates as hydrothermal quartz. The model predicts that the observed decrease in δ30Si values (+0.66 to -1.01 ‰) compared to the isotopic values characteristic for basaltic rocks and meteoric water (+0.68 ‰, Georg et al., 2007a

De La Rocha, C.L., Brzezinski, M.A., DeNiro, M.J. (2000) A first look at the distribution of the stable isotopes of silicon in natural waters. Geochimica et Cosmochimica Acta 64, 2467-2477.

) results from isotope fractionation upon progressive fluid-rock interaction, boiling and cooling. In contrast, oxygen in quartz has multiple sources with the δ18O values dominated by the source water at low rock-water ratio (ξ < 0.1 mol basalt/kg water) and the primary rock at high rock-water ratio (ξ = 0.1-10 mol basalt/kg water; Figs. 3, S-4). The δ18O value of the source water also varies. Seawater can be approximated to a value of 0 ± 1 ‰ (vSMOW), while meteoric water will depend on the geographical location, with values of -7.5 ± 1 ‰ in the south but -12.5 ± 1 ‰ in the north of Iceland (Árnason, 1976

Figure 3 The conceptual chemical and isotope model involves (a) fluid-rock interaction with various source fluids and (b) boiling followed by cooling and mineral formation; (c) isotope systematics in hydrothermal high-temperature quartz largely match the predicted reaction path for progressive fluid-rock interaction (shaded areas) and boiling (arrows). δ30Si decreases with increasing rock-fluid ratio (ξ) whereas δ18O values in quartz both depend on the source water and rock-fluid ratio; (d) most of the isotope variations in hydrothermal low-temperature quartz and silica polymorphs can be explained by boiling and cooling of a hydrothermal fluid (arrows). Silica deposits with low δ30Si and constant δ18O values may result from open system boiling of a high-temperature fluid followed by cooling and quartz or opal formation. BAS = basalt, mw = meteoric water, sw = seawater. Analytical uncertainties of δ30Si and δ18O are listed in Tables S-7 and S-8.
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Hydrothermal low-temperature quartz and silica polymorphs are typically formed by precipitation of dissolved silica upon cooling at or near the surface (e.g., Neuhoff et al., 1999

). The trends observed in Figure 3d are consistent with these silica phases having δ30Si values similar to, or more negative than, the primary host rocks. The depletion of 30Si in the silica minerals cannot be explained by simple fluid-rock interaction and equilibrium fractionation. According to our model, equilibrium fractionation associated with fluid-rock interaction would result in precipitation of quartz with δ30Si values ranging from +2 to +0.5 ‰ and, depending on the fluid source, δ18O values of +10 ± 5 ‰ or +15 ± 5 ‰. Instead, the observed δ18O and δ30Si values follow a trend predicted by boiling of high-temperature hydrothermal fluids to surface (100 °C) followed by cooling (Fig. 3d). The model predicts that the fluid becomes progressively more negative in δ30Si than the corresponding silica minerals. However, recent measurements of silica precipitates and coexisting hydrothermal water suggest the opposite, i.e. that the boiled water is enriched in 30Si relative to the solid precipitate (Geilert et al., 2015

), this suggests that fractionation between silica deposits and fluids under low-temperature (<150 °C) hydrothermal conditions is likely controlled by kinetics and may strongly depend on precipitation rates.

). The results of the present study indicate that such trends are also typical for open system silica precipitation from a hydrothermal fluid conduit and near-surface cooling, where the chemical and isotope composition of the fluid and silicates changes along the fluid flow path (Fig. 4). Differences in δ18O values between silica deposits may not only result from varying temperature conditions but may also derive from variation in the source water. Furthermore, despite the sedimentary origin of cherts and banded iron formations (BIFs), these rocks have often been exposed to hydrothermal alteration (e.g., Van den Boorn et al., 2007

). Thus, the interaction between seawater, hydrothermal fluids and host rocks might have an effect on the Si and O composition even in those silica deposits.

Despite the lack of fractionation factors between fluids and precipitating minerals, this study demonstrates that progressive fluid-rock interaction and the reactions involved may have a strong influence on the oxygen and silicon isotopic characteristics of hydrothermal fluids and associated secondary minerals over a wide temperature range. Fractionation between fluids and minerals of reactive elements like silicon and oxygen are dependent on the source(s) and chemical reactions, aqueous and gas speciation changes as well as secondary mineral formation. Such processes could lead to δ30Si and δ18O variations in hydrothermal silica deposits of >2 ‰ and >20 ‰, respectively, for a system dominated by a single source of both primary rock and water.

). A highly variable but largely negative range in δ30Si values is likely to result from quartz precipitating out of a boiling and cooling fluid involving kinetic fluid-quartz/opal fractionation during rapid temperature decrease. For modelling, the isotope composition of ancient seawater was used (Marin-Carbonne et al., 2014

The δ18O and δ30Si systematics of quartz were determined in situ using SIMS to assess crustal quartz formation processes in Iceland. Magmatic quartz records δ18O of -5.6 to +6.6 ‰ and δ30Si of -0.4 ± 0.2 ‰ representative of mantle- and crustally-derived melts in Iceland. Hydrothermal quartz and silica polymorphs (<150 to 400 °C) record a much greater range with δ18O of -9.3 to +30.1 ‰ and δ30Si of -4.6 to +0.7 ‰. Isotope modelling reveals that these large variations in the δ18O and δ30Si values are caused by a combination of processes such as fluid-rock interaction, cooling, boiling and associated changes in aqueous and gas speciation of the fluids as well as type and quantity of secondary minerals formed upon these processes. Comparison of the results of this study suggests that the large ranges observed in δ30Si values and insignificant δ18O variations observed in quartz cements and quartz amygdales of hydrothermal origin may result from silica formation in a hydrothermal fluid conduit at or near the surface associated with cooling. Equilibrium fractionation of δ18O and δ30Si between fluids and minerals seems to prevail at hydrothermal high-temperature conditions (~200-400 °C), whereas kinetic fractionation is likely to influence isotope systematics at lower temperatures.

Seawater can be approximated to a value of 0 ± 1 ‰ (vSMOW), while meteoric water will depend on the geographical location, with values of -7.5 ± 1 ‰ in the south but -12.5 ± 1 ‰ in the north of Iceland (Árnason, 1976).
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De La Rocha, C.L., Brzezinski, M.A., DeNiro, M.J. (2000) A first look at the distribution of the stable isotopes of silicon in natural waters. Geochimica et Cosmochimica Acta 64, 2467-2477.
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Quartz is among the most abundant minerals in the continental crust and a major constituent of many plutonic, sedimentary and metamorphic rocks (e.g., Götze, 2009).
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Trace element concentrations of quartz agree with existing quartz data from other magmatic and hydrothermal settings (Götze, 2009; Table S-2).
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Marin-Carbonne, J., Robert, F., Chaussidon, M. (2014) The silicon and oxygen isotope compositions of Precambrian cherts: A record of oceanic paleo-temperatures? Precambrian Research 247, 223-234.
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However, as a result of extensive fluid-rock interaction involving a low-18O meteoric water component, strong δ18O-depletions from such a mantle source are commonly observed in Icelandic rocks (Muehlenbachs et al., 1974).
View in article

Hydrothermal low-temperature quartz and silica polymorphs are typically formed by precipitation of dissolved silica upon cooling at or near the surface (e.g., Neuhoff et al., 1999; Geilert et al., 2015).
View in article

Applying isotope modelling approaches (Stefánsson et al., 2017), fluid sources, chemical reactions and associated isotope fractionations were quantified for various processes occurring in the curst, including fluid-rock interaction, fluid phase separation (boiling) and cooling.
View in article
Such processes lead to changes in the relative abundance of elements of various sources, e.g., the water to rock ratio, changes in aqueous species and gas concentrations and quantity of secondary minerals formed, which may all influence isotopic characteristics of fluids and minerals (Stefánsson et al., 2017).
View in article

).
Back to article | Download in PowerpointFigure 3 The conceptual chemical and isotope model involves (a) fluid-rock interaction with various source fluids and (b) boiling followed by cooling and mineral formation; (c) isotope systematics in hydrothermal high-temperature quartz largely match the predicted reaction path for progressive fluid-rock interaction (shaded areas) and boiling (arrows). δ30Si decreases with increasing rock-fluid ratio (ξ) whereas δ18O values in quartz both depend on the source water and rock-fluid ratio; (d) most of the isotope variations in hydrothermal low-temperature quartz and silica polymorphs can be explained by boiling and cooling of a hydrothermal fluid (arrows). Silica deposits with low δ30Si and constant δ18O values may result from open system boiling of a high-temperature fluid followed by cooling and quartz or opal formation. BAS = basalt, mw = meteoric water, sw = seawater. Analytical uncertainties of δ30Si and δ18O are listed in Tables S-7 and S-8.
Back to article | Download in PowerpointFigure 4 δ30Si versus δ18O values of ancient, hydrothermally altered quartz cement and amygdales (Brengman et al., 2016

). A highly variable but largely negative range in δ30Si values is likely to result from quartz precipitating out of a boiling and cooling fluid involving kinetic fluid-quartz/opal fractionation during rapid temperature decrease. For modelling, the isotope composition of ancient seawater was used (Marin-Carbonne et al., 2014